Interest in catalysis continues to grow in the polyolefin industry. Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. To improve polymer properties, single-site catalysts, in particular metallocenes are beginning to replace Ziegler-Natta catalysts. Single-site catalysts typically require large amounts of expensive activators such as methylalumoxane or salts of non-nucleophilic anions such as triphenylcarbenium tetrakis(pentafluorophenyl)borate. It would be desirable to incorporate some of the advantages of single-site catalysts, such as narrow molecular weight distribution and good comonomer incorporation, without the high cost due to the activators.
One useful Ziegler-Natta catalyst is disclosed in U.S. Pat. No. 4,464,518. A first component is made by reacting an organoaluminum compound with a mixture of linear and branched alcohols. Separately, a second component is made by reacting an organomagnesium compound with a silicon-containing compound. The first and second components are combined and then further reacted with a halogen-containing vanadium or titanium compound. Reaction of this product with another organoaluminum compound gives the final catalyst. The catalyst is described as being useful for ethylene polymerizations and ethylene copolymerizations with alpha-olefins such as propylene, 1-butene, 1-hexene or 1-octene. Single-site catalysts are not used. No chelating ligands are used. The catalyst is reported to yield polyethylene powder with a high bulk density.
Single-site catalysts typically feature at least one polymerization-stable, anionic ligand that is purely aromatic, as in a cyclopentadienyl system. All five carbons in the planar cyclopentadienyl ring participate in η-5 bonding to the metal. The cyclopentadienyl anion functions as a 6π-electron donor. Similar bonding apparently occurs with heteroatomic ligands such as boratabenzenyl or azaborolinyl.
Single-site catalysts are used with expensive activators such as alumoxanes, anionic compounds of boron, trialkylboron, and triarylboron compounds.
U.S. Pat. Nos. 5,459,116, 5,798,424, and 6,114,276 teach olefin polymerization catalysts that do not require the use of expensive activators. They employ chelated titanium compounds. Similarly, Eur. Pat. No. 1,238,989 and U.S. Pat. No. 6,897,176 do not require the use of expensive activators. The catalyst is a Lewis acid and a chelated transition metal compound containing 2 or more atoms selected from boron, nitrogen, oxygen, phosphorus, sulfur, and selenium. The Lewis acid may be a magnesium compound such as magnesium chloride, a dialkylmagnesium, a dialkoxymagnesium, an alkylmagnesium halide, an alkoxymagnesium halide, magnesium metal, or reduced magnesium compounds with a polysiloxane compound. The magnesium compound does not contain a chelating ligand and when an organoaluminum compound is used, it does not contain a chelating ligand. Polymethylhydrosiloxane is not used.
Vanadium (III) and titanium (III) amidinate complexes have been combined with a support prepared from MgCl2. 2.1EtOH and triethylaluminum (Polym. Int. 54 (2005) 837). The catalyst is combined with triisobutylaluminum and ethylene to give polyethylene with a low polydispersity in this single-site catalyst system. Neither a polymethylhydrosiloxane nor an organic magnesium halide is used, and the aluminum compounds used do not contain chelating ligands.
Transition metal complexes containing chelating ligands are known. U.S. Pat. No. 5,637,660 describes transition metal complexes containing chelating ligands based upon pyridine or quinoline. U.S. Pat. No. 6,204,216 describes single-site transition metal complexes based upon amine derivatives such as alkoxyamines. Science and Technology in Catalysis (2002) 517 describes transition metal complexes based upon phenoxyimines supported on magnesium chloride. While transition metal complexes containing chelating ligands are known, apparently polymerization catalysts incorporating a chelating ligand into an organomagnesium halide or an organoaluminum compound have not been contemplated. This has many advantages in ease of preparation of a wide variety of catalyst systems.